Ceramics are generally defined as metal oxides used in making structural articles and electrical devices. Such oxides include TiO.sub.2, ZrO.sub.2, Al.sub.2 O.sub.2, Y.sub.2 O.sub.3, BeO, MgO, SiO.sub.2 and the rare earth oxides such as Ce.sub.2 O.sub.3, and La.sub.2 O.sub.3. Also considered to be ceramics are mixed oxides which include not only mixtures of structural oxides, but hydroxometallate compounds such as silicates, aluminates, titanates, etc. including hydrometallates of CaO, SrO and BaO. In recent years the definition of ceramics has been broadened to include carbides, nitrides and borides, as for example SiC, Si.sub.3 N.sub.4, AlN, ZrB, etc.
For centuries, ceramic materials have been used for dishes, pots and tiles. More recently, these items were made by forming the desired article from a mixture of ceramic particles and a binder-deflocculant processing aid. The article was then fired in a kiln.
The processing aids used with the ceramics to make pots, pottery, etc. contain various contaminants which do not alter the useful properties of the articles which are used at relatively low temperatures, i.e., well below 1000.degree. C. However, such contaminants seriously weaken ceramic materials which would be used at temperatures above 1000.degree. C.
Within the last decade, advances in apparatus such as turbine engines are based, in part, on operation at higher temperatures. Thus, the turbine blades of a turbine engine may be subjected to high temperatures above 1000.degree. C. As metal blades cannot withstand these temperatures, ceramic materials are now being investigated for these and other high temperature structures.
In making high temperature structures such as turbine blades, a ceramic material is mixed with a binder-deflocculant, formed into the desired shape and then fired to sinter the ceramic material into a cohesive body. Prior art binder-deflocculant processing aids include alkali metal salts of organic fatty acids which are salts of strong bases, the most common being Na salt of an organic fatty acid or a polymeric acid. Sodium or potassium alkylsulfates are also utilized as binder-deflocculants.
The problem with the prior art processing aids is that after the ceramic article is fired at a temperature great enough to sinter the ceramic material, a contaminant such as sodium, potassium or other metal residue of the processing additive remains. Firing the article at a still greater temperature will drive off some of the contaminants but there is always a residual amount which has a deleterious effect on the high temperature strength of the article.
U.S. Pat. No. 4,562,050 to Koeda et al discloses a process for producing boron nitride of high purity by employing guanidine or similar compounds, guanidine chloride, a weak base, being specifically mentioned.
U.S. Pat. No. 4,800,183 to Quinby discloses the making of a nitride powder by heating a solvent containing a metal salt and melamine until a metal-melamine precipitate forms. The precipitate is calcined at a temperature below 700.degree. C.